CMake: improve linking OpenGL3 example programs (#959)

- Remove find_library(LIB_GLEW ...) from 'resources.cmake'

- Use the variable FLTK_GLEW_DIR to "find" a user supplied GLEW library
  for OpenGL3 example programs (examples/OpenGL3*.cxx).
  Both static and shared libs (DLLs) are supported, but the static
  library is preferred to avoid DLL dependencies.

- Define preprocessor macro 'GLEW_STATIC' by CMake compile definitions
  if and only if the static glew library (glew32s) was found and linked.
  The previous code defined 'GLEW_STATIC' unconditionally in the source
  code which led to undefined references if a shared lib (DLL) was used.

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